1. Field of the Invention
This invention relates to a method and apparatus for the measurement and calibration of fluid flow through work pieces having one or more apertures and, in particular, to a method and apparatus for testing such work pieces and verifying that the apertures have been adequately formed.
2. Related Background Art
It is well known to form apertures, bores, etc. in work pieces, such as gas turbine blades and vane cooling holes, fuel nozzles, combustion chamber cooling holes, and the like. A variety of different processes are used to form such apertures, including casting, mechanical machining, for example drilling, electrical machining, for example electrical discharge machining (EDM), electrochemical machining such as capillary drilling, or combination of such processes.
It is usually desirable and, in some applications, essential to verify that such aperture or bores have been correctly formed so that they provide the desired amount of fluid flow under specified conditions, thereby ensuring correct operation of the work piece in question and minimizing the probability of failure of a component during use.
The present invention is based on a modification of a known type of airflow test system for the above purpose and in order to fully understand the invention, the known system will now be described with reference to FIGS. 1 and 2 which show an embodiment of the present invention including parts of the known system. The known system is based on the use of critical flow nozzles 100,102. The system comprises an air inlet 104 through which pressurized air, typically at 7 Bar absolute (for example from a factory compressor or the like) passes to an air/water filter 106 via a ball valve 108. The air then passes to an accumulator 110, which may be a carbon steel receiver typically of 127 liter capacity, and from there to a fine particle/oil filter 112. Thus, pressurized clean, dry air passes to the critical flow nozzles 100,102 via a pilot operated pressure regulator 114. The pilot operated pressure regulator 114 can be adjusted to control the pressure P1A at the inlet to the critical flow nozzles 100,102, and thus the mass flow rate through the nozzles 100,102 to a test station 116. Airflow to the test station 116 is selectively controlled via respective ball valves 118,120, and a pressure relief valve 122 is provided at the inlet to the test station 116, to protect the work station pressure transducers from damage due to accidental over pressurization.
FIG. 2 is a schematic view of the system of FIG. 1, with the air/water filter accumulator and fine particle/oil filter omitted. Further, for clarity, only one of the critical flow nozzles 100 is shown. Thus, in use, pressurized clean, dry air flows through the critical flow nozzles 100,102 via a pressure regulator 114. The absolute air pressure P1A and the temperature T1 at the inlet of the critical flow nozzles 100,102 are measured and the mass flow of air from the outlet of each critical flow nozzle 100 can be calculated using the equation:   WFN  ⁢      xe2x80x83    =            (              K        ·                  xe2x80x83                ⁢        P1A            )              T1      
where:
WFN=flow nozzle mass flow;
P1A=absolute air pressure;
T1=absolute temperature; and
K=flow nozzle calibration constant (usually provided for the nozzle by its manufacturer).
The total mass flow through the critical flow nozzles 100,102 is the sum of the mass flows calculated for each of the critical flow nozzles through which air is flowing, i.e. the nozzles whose respective valves 118,120 are open.
This known total mass airflow then passes to the work test station 116, which typically includes a flow straightener 124. The work test station 116 is designed to support, seal and clamp the work piece 126 to be tested so that all of the air from the critical flow nozzles 100,102 passes through it, but it will be apparent to persons skilled in the art that such supporting, sealing and clamping arrangements (not shown) will be different for each type of test piece, as each type has its own specific requirements.
The gauge air pressure P3G and the absolute temperature T2 are measured at the inlet to the test piece 126, as is the absolute (barometric) air pressure PA of the air as it exits the test piece 126. It will be appreciated that the absolute pressure PA of the air as it exits the test piece 126 will be atmospheric pressure if the system vents to atmosphere.
The pressure ratio PR can be calculated using the following equation:   PR  =            PA      +      P3G        PA  
and various test piece characteristics can be determined. For example, the effective area of the test piece can be calculated using the following equation:   AEFF  =      WTP                  2.        ⁢                  D2          .                      (            P3G            )                              
where:
AEFF=effective area of test piece;
D2=test piece inlet density; and
WTP=test piece mass flow=A.CD {square root over (2.D2.G.P3G)}
where:
A=total discharge area of test piece;
CD=discharge coefficient for test piece; and
G=gravitational constant.
This assumes that the cross-sectional area of the flow straightener is sufficiently large compared to the test piece cross-sectional area that the total absolute pressure at P3G tapping can be assumed to be equal to the static absolute pressure (P3G+PA), i.e. the flow velocity at the tapping is very low. If this is not the case the equation needs to be corrected for the dynamic pressure (kinetic head).
Thus, with kinetic head correction, this becomes:   AEFF  ⁢      xe2x80x83    =      xe2x80x83    ⁢      WTP                  2.        ⁢                  D2          .                      (            PD            )                              
where:
PD=test piece total differential pressure drop, i.e. including dynamic pressure.
The flow parameter of the test piece can be calculated using the following equation:   FP  ⁢      xe2x80x83    =      xe2x80x83    ⁢                    WTP        ⁢                  xe2x80x83                ⁢                  T2                    P1        =          AEFF      ⁢                                    2.            ⁢                          (                              PR                -                1                            )                                            PA            .            R                              
where:
R=gas constant
FP=flow parameter of test piece; and
P1=test piece absolute inlet pressure.
There are, in fact, a wide range of test piece characteristics which can be measured, and those chosen to be measured and/or calculated within any particular system are dependent upon user requirements.
A typical test specification requires the fluid pressure at the inlet to the test piece 126 to be adjusted to a particular pressure ratio (or equivalent parameter), and then the desired characteristics of the test piece to be determined, for example, the effective area, discharge coefficient, flow parameters, etc.
In conventional systems, the desired pressure ratio is obtained by manual or automatic adjustment of the fluid flow rate through the test piece. It will be appreciated that in a typical test, where the test piece 126 is vented to atmosphere, the inlet pressure required to give the desired pressure ratio depends on the atmospheric (barometric pressure) and therefore with time. Further, the altitude at which the test is conducted can be very significant. In any event, it is relatively difficult to achieve a stable exact setting, and a setting tolerance is therefore allowed. Even then, manual setting is quite skilled and time consuming. In addition, as the flow characteristics of a typical test piece are quite often very sensitive to pressure ratio (due, for example, to the complexity and variations in size of their internal passages) the error due to incorrect setting can be very significant, for example, +1-0.5% compared to an overall error budget of 1%.
We have now devised an arrangement which overcomes the problems outlined above.
Thus, in accordance with a first aspect of the present invention, there is provided fluid flow measurement apparatus for verifying one or more apertures in an object, such as a work piece, the apparatus comprising a source of pressurized fluid and adjustment means for adjusting the fluid flow from the source, means for measuring said fluid flow, means for mounting or otherwise arranging a test piece in the fluid flow from the source such that fluid flows through the at least one aperture therein, means for measuring at least one test piece characteristic at each of a plurality of test conditions in a range, said range including a predetermined desired test condition, and processing means for calculating a mathematical function or equation derived from said plurality of measured test piece characteristics and test conditions and determining from said mathematical function or equation the test piece characteristic which corresponds to said predetermined desired test condition.
Also in accordance with the first aspect of the present invention, there is provided a method of verifying one or more apertures in an object, such as a work piece, the method comprising the steps of providing a source of pressurized fluid, means for measuring fluid flow from the source and adjustment means for adjusting said fluid flow, mounting or otherwise arranging a test piece in the fluid flow from the source such that fluid flows through the at least one aperture therein, measuring at least one test piece characteristic at each of a plurality of test conditions in a range by adjusting said fluid flow from the source, said range including a predetermined desired test condition, calculating a mathematical function or equation derived from said plurality of measured test piece characteristics and test conditions and determining from said mathematical function or equation the test piece characteristic which substantially exactly corresponds to said predetermined desired test condition.
Thus, the test piece characteristic can be determined at the desired test condition to a high degree of accuracy without the need to set the measured test condition precisely at the desired condition.
According to a preferred embodiment, the first aspect of the invention provides a method of verifying one or more apertures in a work piece, the method comprising the steps of:
connecting an adjustable source of fluid flow to the test piece,
adjusting the fluid flow until a measured test condition (e.g. pressure ratio) is near a predetermined desired value, allowing the flow conditions to stabilize if necessary,
recording the measured test condition and a measured test piece characteristic (e.g. effective area) at said predetermined desired test condition,
adjusting the fluid flow a plurality of times so that the measured test condition passes either side of said predetermined desired test condition and recording the values of the test condition and corresponding test piece characteristic each time,
processing the recorded data to produce a mathematical function or equation for test piece characteristic versus test condition and using the function or equation to determine the test piece characteristic at said predetermined desired test condition.
The preferred embodiment of the first aspect of the invention extends to an apparatus having means for carrying out each of the above method steps.
Preferably the test condition is the test piece pressure ratio and the test fluid is preferably air.
A control computer (not shown) is provided in the system of FIGS. 1 and 2, and all of the pressure and temperature measurements are read by the control computer, via, for example, a RS232 Serial interface connected to the measurement channels. The control computer may also control the pilot valve of the pilot operated pressure regulator 114 and valves, depending on the system configuration, although in other cases, these are operated manually.
The control computer also provides facilities for creating, modifying and storing test procedures and specifications, selecting a procedure created previously, executing a test procedure selected previously, storing, printing and displaying test results, etc.
Before a flow test can be performed, a test procedure must be created using the test procedure creation facilities on the control computer to define the system settings to perform the test. A test procedure usually defines the critical flow nozzle channels to be used and the pressure ratios to be set. In conventional systems, these are usually derived from a test specification prepared by the engineer who designed the test piece 126.
However, in some cases, no prior data exists and the procedure must be generated by trying the part on the system to establish the most suitable flow channels, pressure ratios, and other parameters to use. In other cases, the available data is in different units to that used on the airflow system. As a result of these, and other factors, it can often be a time consuming and skilled task to create a satisfactory test procedure for a new unknown test piece.
Thus, in accordance with a second aspect of the present invention, there is provided fluid flow measurement apparatus for verifying one or more apertures in an object, such as a work piece, the apparatus comprising a source of pressurized fluid, means for mounting or otherwise arranging a test piece in the fluid flow from the source such that fluid flows through the at least one aperture therein, measurement means for measuring at least one test piece characteristic at at least one test condition, and means for automatically creating a test procedure for a test piece, said test procedure at least including a desired test condition for said test piece.
A preferred embodiment of the second aspect of the invention includes one or more of the following features:
intelligent test procedure editor which uses the critical flow nozzle equations to xe2x80x98buildxe2x80x99 the test procedure from basic test piece data information,
automatic conversion between different units,
a self programming operating mode with procedures that either provide operator prompts to interactively flow a test piece on the system and create a suitable test procedure, or if the system is fitted with automatic valves to automatically flow a test piece on the system and create a suitable test procedure,
a standard automatic test cycle that can be performed on an unknown test piece to obtain its flow characteristic over a full range of flow conditions.